This invention relates to emission control for coal-fired electric power plants and, more particularly, to a new and improved process which employs an integrated injection and fabric filter baghouse system for simultaneous SO.sub.x, NO.sub.x, and particulates control.
Advanced control technologies have rapidly evolved during the past two decades for dealing with coal-fired power plant emissions of particulates, sulfur oxides and nitrogen oxides.
A dramatic shift to low-sulfur coals, which produce high-resistivity fly-ashes, that are more difficult to collect, and the concurrent demand for increased particulate efficiency, placed a heavy burden on conventional dust collectors (electrostatic precipitators) employed in these applications. Consequently, in the late 1970's the electric utility industry began, on a significant scale, to utilize alternative technology--fabric filterhouses--for particulate control.
In a bag filterhouse, fly-ash is separated from flue gas by filtration, i.e. the fly-ash is collected on the upstream side of the filter bags as the gas is directed through tubular shaped fabric filter bags. The flue gas passes through the bag, typically from the inside out, and the fly-ash is collected as so-called filter cake. The material collected on the bag becomes part of the filtering medium. After a certain build-up of filter cake over the course of operation, the bags must be cleaned to avoid excessive pressure drop in order to maintain proper gas volume flow. The use of a bag filterhouse gives the utility the option of switching coals since bag filterhouse collection efficiency is relatively insensitive to fuel characteristics.
Nevertheless, as coal burns, most of the sulfur content is converted to sulfur oxides (SO.sub.x) , typically SO.sub.2 and SO.sub.3. In addition, oxides of nitrogen (collectively referred to as NO.sub.x) are formed. The utilization and placement of NO.sub.x and SO.sub.x control equipment upstream of the bag filterhouse will influence inlet conditions. Overall location of the bag filterhouse relative to such equipment, as well as the plant air heaters, flue gas temperature, flue gas composition, and fabric filter specifications including material properties such as composition, tensile strength, abrasion resistance, chemical resistance and temperature limitations comprise the major parameters which must be carefully considered, integrated and controlled in order to achieve satisfactory overall plat emissions control.
In U.S. Pat. No. 4,309,386, which is assigned to the assignee of the present invention, a hot catalytic baghouse (greater than 600.degree. F. was employed for simultaneous particulate removal and NO.sub.x reduction. In accordance with the teachings of that patent, the filter bags were treated with a catalyst to facilitate the selective catalytic reduction process while simultaneously filtering out particulate matter form the gas stream. The baghouse was situated downstream form an ammonia injection system. In the baghouse, the flue as stream was exposed to the treated bags to effect NO.sub.x removal. However, that system has several potential shortcomings. In particular, bags impregnated with the catalyst tend to have a limited life span, necessitating periodic replacement of the entire bag and the catalyst is subject to SO.sub.x poisoning which can lead to loss of ability to reduce NO.sub.x in the present of gases containing high quantities of SO.sub.x.
Therefore, continuing improvements are being sought in emission control systems employing catalytic fabric baghouses with satisfactory control of SO.sub.x and NO.sub.x emissions.